In vitro estimation of aortic QO2in normal rabbits revealed the thoracic portion to be lower than that of the arch and abdominal segments. These latter were not significantly different. Renal hypertension, cortisone administration and their combination with or without cholesterol feeding resulted in significant elevations of the oxygen consumption of all segments. The thoracic portion under these conditions was lowest and that of the arch was greater than that observed in the abdominal aorta. A depression of QO2was observed in the aortas from normotensive rabbits subjected to cholesterol feeding with or without cortisone treatment.Although these findings indicate that hypertension and cortisone may profoundly alter the metabolic activity of the aorta, they could not be correlated with the degrees of cholesterol atherosclerosis observed in the various experimental situations explored.